Adds documentation for #629

Documents the new `$layout` argument, configuration options, and
defaults for the `ErrorResponseGenerator`. Additionally, provides the
default values for template and layout-related constants defined by both
the `NotFoundHandler` and `ErrorResponseGenerator`.

Author: weierophinney

docs/book/v3/features/container/factories.md

@@ -81,7 +81,11 @@ When the `config` service is present, the factory can utilize two values:
 - `zend-expressive.error_handler.template_error`, a name of an alternate
   template to use (instead of the default represented in the
   `Zend\Expressive\Middleware\ErrorResponseGenerator::TEMPLATE_DEFAULT`
-  constant).
+  constant, which evaluates to `error::error`).
+- **Since 3.1.0**: `zend-expressive.error_handler.layout`, a name of an
+  alternate layout to use (instead of the default represented in the
+  `Zend\Expressive\Middleware\ErrorResponseGenerator::LAYOUT_DEFAULT` constant,
+  which evaluates to `layout::default`).
 
 As an example:
 
@@ -90,6 +94,7 @@ As an example:
 'zend-expressive' => [
     'error_handler' => [
         'template_error' => 'name of error template',
+        'layout' => 'layout::alternate',
     ],
 ],
 ```
@@ -150,14 +155,21 @@ When the `config` service is present, the factory can utilize two values:
 
 - `zend-expressive.error_handler.template_404`, a name of an alternate
   template to use (instead of the default represented in the
-  `Zend\Expressive\Delegate\NotFoundDelegate::TEMPLATE_DEFAULT` constant).
+  `Zend\Expressive\Delegate\NotFoundDelegate::TEMPLATE_DEFAULT` constant, which
+  evaluates to `error::404`).
+
+- `zend-expressive.error_handler.layout`, a name of an alternate
+  template to use (instead of the default represented in the
+  `Zend\Expressive\Delegate\NotFoundDelegate::TEMPLATE_DEFAULT` constant, which
+  evaluates to `layout::default`).
 
 As an example:
 
 ```php
 'zend-expressive' => [
     'error_handler' => [
         'template_404' => 'name of 404 template',
+        'layout' => 'layout::alternate',
     ],
 ],
 ```

docs/book/v3/features/error-handling.md

@@ -71,6 +71,9 @@ We provide two error response generators for you:
   otherwise, a plain text response is generated that notes the request method
   and URI.
 
+  Since version 3.1.0, it also accepts a layout name, if you want to use one
+  other than `layout::default`.
+
 - `Zend\Expressive\Middleware\WhoopsErrorResponseGenerator`, which uses
   [whoops](http://filp.github.io/whoops/) to present detailed exception
   and request information; this implementation is intended for development